Comet Hale-Bopp Light Curve Update

Mark Kidger
April 3, 1997
mrk@ll.iac.es

The light curve of the comet through perihelion reveals various conflicting
tendencies in estimates. This comet has what must be the best light curve
coverage ever for a comet around perihelion and, with the vast archive of data
since discovery, is going to provide enormous quantities of information for
future research. Here is a quick summary of the situation as of yesterday:

How bright did Hale-Bopp get?

The dispersion in magnitudes at perihelion was very large. The comet was large
and extremely condensed, making estimation very tough. My own peak estimate
was only -0.4, but that was in very poor conditions in England (limiting
magnitude around 4.5). Good observers in very good sites estimated much brighter
magnitudes on the whole. The estimate will depend a lot on how the comparison
was made by individual observers (I ended up using Mars and Capella as my own
comparison stars). This has a complication in that the comet, which is strongly
blue, was a very different colour to the best comparison stars and this can
exagerate the peak magnitude which was estimated (red stars appear rather
fainter to the eye, so comparing with red stars makes the comet seem brighter
than it really is).

The brightest individual estimate that I have seen was -1.6, but this is an
isolated point. From good sites the peak magnitude seems to have been about
magnitude -1.0+/-0.1 with the "average" peak magnitude being around -0.7.

How much brighter than Comet Hyakutake was Comet Hale-Bopp?

Another good question for the reasons given above. Hale-Bopp was definitely
much brighter and will remain brighter for some days than Hyakutake was at
maximum. Most observers estimated a peak magnitude of Hyakutake around +0.5
last year (which lasted only very briefly), hence Hale-Bopp appears to have been
about 1.5 magnitudes (a factor of 4) brighter.

What about the comparison with Comet West?

Comet West reached about magnitude -3.5 very briefly around perihelion. However,
it was not visible to the naked eye at this time (rather like Comet Kohoutek
which also reached about magnitude -3, but was only observable to the naked-eye
for the Skylab astronauts in Earth orbit). Comet West was only a negative
magnitude object for a few days, while Hale-Bopp will remain negative magnitude
for as much as 5 or 6 weeks in total. In this sense, Comet Hale-Bopp is a much
brighter comet. The tail however was much more spectacular and better developed
in Comet West.

How did the light curve develop in the end?

It seems that the linear brightening stopped about 3 weeks before perihelion.
However, many of the observations very close to perihelion are consistent with
an almost unchanged rate of brightening right through. The point where the
light curve most obviously flattens out conicides with the Full Moon in late
March, so those estimates are not necessarily the full brightness of the comet
anyway. Even the faintest estimates (generally the ones by observers who
estimated the magnitude in poorer skies), show the comet still increasing in
brightness steadily THROUGH perihelion. This is a major surprise because, in
theory, the comet should have peaked in late March and then started to decline
noticeably by perihelion. At the moment it is a little hard to tell what
happened to the light curve at perihelion and after because some of the
observations are consistent with both a small outburst at perihelion, followed
by a fade and others with a slow increase in brightness even after perihelion.

How does Hale-Bopp compare intrinsically with other comets of the past?

Very favourably indeed. If Comet Hale-Bopp had passed as close as Comet
Hyakutake did last year it would have been around magnitude -7 and perhaps the
second or third brightest comet since the 17th century! Assuming a maximum
magnitude of -1.0, the absolute magnitude of Comet Hale-Bopp was -1.3. These
are the ten very brightest recorded values:

For comparison, Comet Halley has an absolute magnitude of +4.0 and an "average"
new comet is around +6.5. Comet Hale-Bopp is thus about 130 times brighter than
Comet Halley at the same distance and 1300 times brighter than an "average"
comet would have been.

How long is the tail?

The longest estimates at perihelion, from dark sites were 20 degrees. This
works out at exactly 1.0AU, or 150 million miles. This is a much longer tail
length than for Comet Hyakutake and about 3 times the true length of Comet
Halley's tail. We can expect the tail to grow even longer over the next
couple of weeks.